1. Field of the Invention
The invention relates to nonlinear gain microchannel plate (MCP) image intensifier tubes, and especially to intensifier tubes having MCPs whose resistivity is increased to yield nonlinear gain characteristics suitable for extending the tube's dynamic range and having phosphor screens whose phosphor transfer efficiency is changed to ameliorate undesirable temporal gain effects resulting from the increased resistivity.
2. Description of the Prior Art
Second and third generation wafer image intensifier goggle tubes have been used to view scenes which frequently contain a substantial fraction of bright above-horizon sky along with the much darker below-horizon ground, the latter being the area of interest. Typically, the above-horizon sky is one hundred to eight hundred times brighter than the below-horizon area of interest.
These image intensifier tubes employ an automatic brightness control (ABC) circuit in the tube power supply which acts to preserve user dark adaptation and to prevent tube damage by not allowing total current reaching the tube phosphor screen to exceed a given value. This circuit accordingly acts to reduce overall tube gain so that the average screen brightness stays below a certain predetermined value. For typical scenes which possess small or moderate ranges of brightness the ABC circuit works very well. However, for horizon scenes which possess large extremes of brightness, the bright above-horizon sky areas cause the ABC circuit to reduce tube gain for both the bright and dark scene areas. This overall tube gain reduction seriously degrades the user's capability for resolving detail in the scene dark area since detail can either be completely lost, or be discernible only after a much longer observation period.
Second and third generation wafer image intensifier goggle tubes use MCP electron multipliers. A typical prior art MCP image intensifier is disclosed in U.S. Pat. No. 3,553,518 to Pieter Schagen, patented on Jan. 5, 1971. In linear MCP operation, electrons emitted from a photocathode strike the channel walls near the input, initiating a cascade of secondary electrons which propagates through the channel and increases exponentially with each subsequent strike along the channel wall. All secondary electrons lost to this cascade from the channel surface layer are replenished by the underlying electronic conduction region such that the channel walls charge only slightly and the electric field through the channel remains uniform and constant. As the output current level in each channel increases, however, the conduction region can no longer adequately replace the electrons emitted from the surface layer. Consequently, the surface layer assumes a positive charge and causes the field through the channel to deform. The net effect of this field distortion is an overall reduction in channel electron gain. In fact, this is the point at which the channel becomes sublinear in electron multiplication. As the input current is increased, the channel gain decreases in a characteristic fashion. It is clear that the gain cannot decrease below unity on a steady-state basis, or else the channel would become an absorber of electrons and would dissipate the accumulated positive charge. However, for high input electron levels, an instability arises in the channel. This instability probably results from the fact that the secondary emission yield of the first electron impact is always greater than unity. For sufficiently high inputs, however, this first strike yield cannot be sustained in that the average channel gain must approach unity as the input current increases. Consequently, the channel gain must oscillate as a function of time. This channel instability cycle causes the net electron multiplication of the channel to continue falling asymptotically towards unity as input electron levels increase and consequently, the time-averaged net electron multiplication from the channel can fall below the value of first strike secondary yield. The frequency and amplitude of this field instability cycle are dependent upon the input electron levels and the resistivity of the channel.
Saturation characteristics, such as those just explained, were measured for several MCP types over a range of electron gains and strip current densities. Typical characteristics are as shown in TABLE I:
TABLE I ______________________________________ Strip Current Density (Amperes/cm.sup.2) Electron MCP Type At Typical Voltages Gain FOM .gamma. ______________________________________ Unfilmed 5 .times. 10.sup.-9 -2 .times. 10.sup.-7 200-1000 .4-.6 .2-.4 Filmed 4 .times. 10.sup.-7 800 .55 .3 Filmed 1-3 .times. 10.sup.-7 200-900 .2-.3 .4 Funneled Coated ______________________________________
Specific characteristics of the MCP types were as follows. The filmed MCP had an approximately 50 .ANG. layer of either aluminum oxide or silicon dioxide placed over the input channel openings to prevent ion feedback. The filmed funneled coated MCP had, in addition to the above, its channel walls thinned at the input to increase the fraction of signal electrons entering the channels and had a coating of high secondary emission material, i.e., magnesium oxide, placed on input channel surfaces to improve electron multiplication. It should be noted that the figure-of-merit (FOM) of the MCP saturation characteristic is defined as a particular output current density Jout value at the knee of a characteristic curve, i.e. the Jout value at the intersection of the extrapolations of the linear gain and sublinear region as discussed herein above, divided by strip current density J.sub.st which represents the channel wall conduction current per unit area, expressed in amps/cm.sup.2. The gamma characteristic .gamma. represents the log-log slope of the sublinear region, and assumes a value between zero and unity.
Initial attempts to utilize nonlinear gain characteristics caused by substantially increased resistance of the prototype MCPs resulted in undesirable temporal characteristics. These temporal characteristics were not only cosmetically distracting, but also severely degraded tube imagery. The temporal effects that are the primary design constraints are categorized as follows.
The first temporal effect is gain recovery time. This time effect manifests itself as a band which appears when the tube is scanned across a bright/dark interface of a scene. The band originates at this interface and may be either brighter or darker than its surroundings, depending on the direction in which the tube is scanned across the interface. The width of the band is proportional to the rate of the scan. The appearance of this transient response is a shimmering or waviness of bright/dark interfaces which can readily induce discomfort and/or disorientation in the user.
A second temporal effect problem is bright borders. This effect appears as a bright outline which forms at a stationary, sharply-demarcated bright/dark interface. The bright outline forms over the course of approximately one minute, and if the bright/dark interface is displaced after this period the bright border will persist in its original position for over a minute before fading away. Although this time effect is very noticeable in static laboratory tests, it is not evident in field tests. This is because the bright/dark interfaces in the field are neither sharp enough nor of sufficiently high contrast to induce the effect, and also because there is always enough image motion in the field environment to prevent the burn-in of interface outlines required for the effect to occur.
Travelling waves are a third temporal effect problem. This problem only occurs when the tube is scanned over a bright/dark interface such that a dark area is moved into a previously bright area on the tube. It manifests itself as several dark bands or waves which originate in the bright region on the tube and travel toward the interface for one or two seconds after the scan is completed. This is perhaps the most serious time effect because it occurs very frequently in typical horizon scenarios and can be extremely distracting to the user.
Rippling is the fourth problem. This problem occurs only when there are very bright light sources in the scene, and it manifests itself as a steady movement of dark ripples or waves across the bright areas. Unlike travelling waves, the rippling requires no image motion or scanning to be initiated, and it persists for as long as a sufficiently bright area remains on the input of the tube. Since there are many very bright sources in typical field scenarios (e.g., headlights, floodlights, flares) rippling constitutes a definite design constraint.
These limitations, disadvantages and problems inherent in the prior art are overcome by the present invention.